The present invention relates to forming a platinum modified diffusion aluminide coating on a superalloy component, such as a gas turbine engine blade and vane, exposed to high service temperatures.
Advancements in propulsion technologies have required gas turbine engines to operate at higher temperatures. This increase in operating temperature has required concomitant advancements in the operating temperatures of metallic (e.g. nickel and cobalt base superalloy) turbine engine components to withstand oxidation and hot corrosion in service. Inwardly grown and outwardly grown platinum modified diffusion aluminide coatings have been formed on superalloy turbine engine components to meet these higher temperature requirements. One such inwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using aluminide halide coating gas and comprises an inward diffusion zone and an outer two phase [PtAl2+(Ni,Pt)Al] layer. The two phase Pt modified diffusion aluminide coatings are relatively hard and brittle and have been observed to be sensitive to thermal mechanical fatigue (TMF) cracking in gas turbine engine service.
One such outwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using a low activity aluminide halide coating gas as described in U.S. Pat. Nos. 5,658,614; 5,716,720; 5,989,733; and 5,788,823 and comprises an inward diffusion zone and an outer (additive) single phase (Ni,Pt)Al layer.
An object of the present invention is to provide a gas phase aluminizing method using one or more solid sources of aluminum for forming on a substrate surface an outwardly grown, single phase diffusion aluminide coating that includes an outer additive layer having a graded Pt content from an outer toward an inner region thereof.
The present invention involves forming on a substrate, such as a nickel or cobalt base superalloy substrate, a platinum modified diffusion aluminide coating by depositing a layer comprising platinum on the substrate and then gas phase aluminizing the substrate in a coating chamber having a solid source of aluminum (e.g. aluminum alloy particulates) disposed therein close enough to the substrate surface as to form at an elevated coating temperature an outwardly grown diffusion aluminide coating having an inner diffusion zone and outer, single phase (Ni,Pt)Al additive layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone. Gas phase aluminizing can be conducted with or without a prediffusion of the platinum layer into the substrate.
The present invention also envisions forming on a substrate a platinum graded, single phase diffusion aluminide coating at a first surface area of the substrate and concurrently a different diffusion aluminide coating at a second surface area of the substrate in the same coating chamber.
The present invention is advantageous to form on a nickel or cobalt base superalloy substrate an outwardly grown platinum modified diffusion aluminide coating having an outer, single phase (Ni,Pt)Al additive layer with a Pt content that is relatively higher at an outermost coating region than at an innermost coating region adjacent to a diffusion zone to impart oxidation and hot corrosion resistance thereto and improved ductility as compared to conventional two phase platinum modified diffusion coatings.
The above objects and advantages of the present invention will become more readily apparent from the following description taken with the following drawings.